The compression of gases is a very important process in many technologies. When compressing or reducing the volume of an ideal or close to ideal gas, heat is produced in addition to an increase in the gas pressure. When the heat produced due to gas compression is removed from the compressing gas by, for example, heat exchange with the surroundings, the process is isothermal.
The expansion of a gas is a process opposite to the process of compression. During the expansion, the gas pressure is decreased and heat is consumed by the expanding gas. In order to achieve isothermal conditions, the amount of heat consumed by the expanding gas is supplied, for example, by heat transfer from the surroundings to the expanding gas.
Gas compression/expansion is commonly used for the storage of energy in compressed air energy storage systems (CAES), the isothermal regime allows the energy loss to be minimized, and therefore, maximizes the overall storage efficiency.
True or theoretical isothermal compression/expansion is difficult if not impossible in actual practice. To achieve true or theoretical isothermal expansion/compression there is a requirement for a zero temperature difference between the compressed/expanded gas and the surroundings. That requires either an infinite heat transfer area, or infinite heat transfer time or both. The real compression/expansion processes can approach the theoretical isothermal compression/expansion to varying degrees. The term pseudo isothermal compression is used here to describe a compression which is between isentropic and truly isothermal. In pseudo isothermal compression some heat is removed from the compressed gas, but it is less than the amount of heat to be removed for truly isothermal compression.
Recently, a process and apparatus for the pseudo isothermal compression and expansion of a gas was disclosed in PCT Applications PCT/CA2013/050972 and PCT/CA2015/050137. The prior art references show processes for compression and expansion that are based on the use of a liquid, which is pumped into a gas/liquid compression device and pushed out from a gas/liquid expansion device. The liquid plays the role of a “liquid piston”. In the prior art reference the liquid and the compressed/expanded gas are in direct contact, i.e. there is a gas-liquid interface. In these disclosures, the heat is transferred from the compressing gas to the surroundings by one or any combination of the following mechanisms. The heat is transferred directly from the compressing gas through the walls of the compression device to the surroundings. The heat is transferred indirectly first from the gas to the liquid piston through their interface and then from the liquid to the surroundings. The heat is transferred indirectly first from the gas to a solid heat sink, then from the solid heat sink to the liquid, and finally from the liquid to the surroundings. Further, the heat transfer mechanisms are the same during expansion, but the heat travels in the opposite direction (from the surroundings towards the expanding gas).
As stated above, in the prior art references there is direct gas-liquid contact. The existence of a liquid surface, contacting the gas, leads to several problems. Some of these problems are listed below. The dissolution of the gas in the liquid (during the increase in the gas pressure), followed by a release of the dissolved gas from the liquid and formation of gas bubbles (during the decrease in the gas pressure), results in a decrease of each of the compression and expansion efficiency. The loss of part of the liquid (forming the liquid piston) from the compression/expansion device together with the gas exiting the compression/expansion device due to (but not limited to) the motion of the liquid in the compression/expansion vessel when waves and other types of motion of the liquid surface are formed; and the foaming of the liquid. This results in the decrease of the compression/expansion efficiency, and also in the loss of liquid from the compression/expansion device, when a part of the liquid is expelled from the compression/expansion device together with the compressed or expanded gas. The foaming can be a result of two main processes: entrainment of gas bubbles in the liquid through the gas-liquid interface; and formation of gas bubbles when the dissolved gas is released from the liquid due to the pressure decrease and/or temperature increase. This process is similar to the formation of gas (carbon dioxide) bubbles in a carbonated drink when a bottle is opened and the pressure above the drink is decreased.
Accordingly it would be advantageous to provide a variable pressure vessel that provides an improved heat transfer mechanism.